1. Field of the Invention
The present invention relates to a liquid crystal panel driving circuit. More particularly, the present invention relates to a liquid crystal panel driving circuit, in which a distortion of a wave form of an output or an oscillation of an operational amplifier for driving a liquid crystal panel that is caused by an influence of a self-inductance of a flexible substrate can be suppressed, when a flexible substrate is used as a medium through which a liquid crystal panel driving circuit and a liquid crystal panel are connected.
2. Description of the Related Art
A liquid crystal panel driving circuit is disposed at an input stage of a liquid crystal panel. This liquid crystal panel driving circuit has a plurality of operational amplifiers corresponding to the number of drive lines. A TFT type liquid crystal panel has transistors (TFTs) arrayed in a form of matrix. Each of the operational amplifiers of the liquid crystal panel driving circuit is connected to, for example, a source of this transistor by the first signal line. Then, the liquid crystal panel serving as a capacitance load is charged and discharged through this transistor. Incidentally, another second signal line is connected to a gate of the transistor. An on/off control of a predetermined transistor is carried out in accordance with an input signal through the second line.
FIG. 8 is a view showing an arrangement of a conventional operational amplifier for a liquid crystal panel. As shown in FIG. 8, an operational amplifier 806a for a liquid crystal panel contains an operational amplifier element 806. The operational amplifier element 806 has a non-inverting input terminal 801 to receive an input signal. An output terminal 802 of the operational amplifier element 806 is connected as a voltage follower to an inverting input terminal 803 so as to form a negative feedback circuit. Typically, the output terminal 802 of the operational amplifier element 806 is connected through a flexible substrate 804 to a liquid crystal panel 805 serving as a load.
Here, the operation of the conventional liquid crystal panel driving circuit is described with reference to FIG. 9. FIG. 9 is a conceptual view of the conventional liquid crystal panel driving circuit. A liquid crystal panel driving circuit 906 shown in FIG. 9 is provided with: an operational amplifier group 901 composed of a plurality of operational amplifiers; a D/A converter group 902 composed of a plurality of D/A converters; and an output terminal group 903 composed of a plurality of output terminals. The D/A converter group 902 is connected to an input stage of the operational amplifier group 901.
The output terminal group 903 is connected to the liquid crystal panel 905 through a flexible substrate group 904 composed of a plurality of flexible substrates.
The operational amplifier group 901 is composed of the operational amplifiers 806a shown in FIG. 8. Thus, the output terminals of the respective operational amplifiers of the operational amplifier group 901 are connected as the voltage follower so as to form the negative feedback circuit.
The operational amplifier group 901 performs an impedance conversion on an analog signal outputted by the D/A converter group 902. The operational amplifier group 901 charges and discharges the liquid crystal panel 905 serving as the capacitance load, on the basis of the signal on which the impedance conversion is performed.
An invention in relation to a drive of a liquid crystal display using an operational amplifier, especially, an invention in relation to a power supply of a liquid crystal display driving circuit is disclosed in Japanese Laid Open Patent Application (JP-A-Showa, 62-83724). The technique disclosed in this gazette is the liquid crystal display driving circuit having the feature of protecting an oscillation of an operational amplifier by adding four resistors to a power supply circuit in which a six-level voltage is generated by using two kinds of power supplies, five resistors and four operational amplifiers. That is, this technique can protect a current increase caused by the oscillation of the operational amplifier. Incidentally, in order to protect the current increase, an output signal of the operational amplifier is outputted through the resistor and supplied to the liquid crystal panel.
Conventionally, the liquid crystal panel driving circuit has been optimally designed by referring to an equivalent circuit of a liquid crystal panel. However, if a reactance component of the flexible substrate is large, the inductance component causes high impedance to be generated in a high frequency band. This high impedance brings about the situation that a signal to be attenuated by the liquid crystal panel is fed back to the liquid crystal driving circuit 906 without any attenuation. For this reason, in the high frequency band, the influence of the high impedance causes the deterioration of the frequency property of the operational amplifier group 901. The deterioration of the frequency property results in the oscillation of the operational amplifier group 901.
Usually, the inductance component has a value of about 0.5 xcexcH, although it is changed depending on the size of the liquid crystal panel. In the case of this condition, the input of a drive signal having a drive frequency of about 200 MHz may result in an occurrence of an oscillation.
The present invention has been made to solve the above-explained problems.
Therefore, an object of the present invention is to provide a liquid crystal panel driving circuit and a method of driving a liquid crystal panel, in which even if an inductance component is added between an operational amplifier and a liquid crystal panel, the operational amplifier is not oscillated.
Another object of the present invention is to provide a liquid crystal panel driving circuit and a method of driving a liquid crystal panel, in which even if an inductance component is added between an operational amplifier and a liquid crystal panel, a speed for driving the liquid crystal panel are not deteriorated.
Still another object of the present invention is to provide a liquid crystal panel driving circuit and method of driving a liquid crystal panel, in which even if an inductance component is added between an operational amplifier and a liquid crystal panel, a static current are not deteriorated.
In order to achieve an aspect of the present invention, the present invention provides a liquid crystal panel driving circuit including an operational amplifier which outputs an output signal based on an image signal and an feedback signal corresponding to the output signal without a high frequency content; and a flexible substrate with an inductance wherein the output signal pass through to a liquid crystal display via the inductance.
In the liquid crystal panel driving circuit, the operational amplifier may includes an operational amplifier element including a non-inverting input terminal supplied with the image signal, an inverting input terminal supplied with the feedback signal and an output terminal outputting the output signal; and a negative feedback circuit formed between the output terminal and the inverting input terminal.
In the liquid crystal panel driving circuit, the negative feedback circuit may include a low pass filter attenuating the high frequency content.
In the liquid crystal panel driving circuit, the low pass filter may includes a resistor connected between the inverting input terminal and the output terminal; and a capacitor connected between the inverting input terminal and an power supply.
In the liquid crystal panel driving circuit, the low pass filter may includes a plurality of resistors connected in series between the inverting input terminal and the output terminal; and a plurality of capacitors connected in parallel to each other through the resistors, wherein one end of the each of plurality of capacitors is connected to a connection point between the resistors and the other end of the each capacitor is connected to a power supply.
In the liquid crystal panel driving circuit, the low pass filter may further includes a plurality of switches, wherein the each of plurality of switches sets an active state and an inactive state of a corresponding one of the plurality of capacitors.
In the liquid crystal panel driving circuit, it may further includes a control circuit controlling an active state and an inactive state of the each switch so as to set a synthesis capacitance of the plurality of capacitors, based on the inductance of the flexible substrate.
In the liquid crystal panel driving circuit, it may further include D/A converter converting a digital image signal to an analog image signal as the image signal.
In order to achieve another aspect of the present invention, the present invention provides a method of driving a liquid crystal panel including (a) amplifying an image signal as a output signal, (b) supplying a liquid crystal display with the output signal; and (c) feeding back an feedback signal based on the output signal without a high frequency content.
In the method of driving a liquid crystal panel, (a) amplifying step includes (d) supplying an operational amplifier element with the image signal and the feedback signal; and (e) amplifying the image signal based on the image signal and the feedback signal by the operational amplifier element.
In the method of driving a liquid crystal panel, (b) supplying step includes (f) supplying the output signal that passes through a flexible substrate with an inductance to the liquid crystal display via the inductance.
In the method of driving a liquid crystal panel, (c) feeding back step includes (g) feeding back the feedback to the operational amplifier element, wherein the high frequency content signal is attenuated.